1. Field of the Invention
The present invention relates to a drill used, for instance, for drilling a hole in an engine component or the like.
2. Description of the Related Art
When aluminum-cast engine components requiring deep hole cutting are manufactured in sequence, a great load is applied on the drill utilized for cutting the components. Accordingly, a drill suitable for cutting a deep hole has conventionally been used during such processing.
Japanese Utility Model Publication No. 53-038953 discloses a drill suitable for performing deep hole processing on metals, where the torsion angle of the entirety or a part of a blade is consecutively changed, and the torsion angle on the distal side is greater than that on the rear end side of the blade. It is mentioned in this document that such an arrangement improves the bite of the drill as well as the rigidity of the drill.
A drill of this type exhibits a variety of scales of strength (e.g., bending strength, rigidity), cutting resistance (cutting reaction force), swarf shapes, and swarf dischargeability, according to the measure of the torsion angle. Accordingly, the lifetime of the drill is influenced by the setting condition of the torsion angle. In particular, a drill used for drilling a deep hole on a workpiece initially having no holes therein is subjected to extremely great loads when drilling the hole in the workpiece. In addition, it is difficult for such a drill to discharge swarfs at a certain depth of the deep hole. Accordingly, the drill may be damaged and become clogged by swarfs.
On the other hand, another problem manifests itself when a workpiece with an existing hole is to be bored along the hole.
For instance, when aluminum-cast engine components are manufactured in sequence, the position of a hole provided on the engine component by casting, i.e., a so-called cast hole, may become deviated according to the casting accuracy. Accordingly, when such a cast hole is to be bored by a drill, the cutting direction (cutting position) of the drill may become misaligned with the cast hole. Such misalignment may cause a great load on the drill, thereby lowering the lifetime and processing accuracy of the tool.
In view of the above, the setting conditions of the torsion angle must be optimized for the drill that is used in processing the cast hole, so that damage to the drill and deterioration of processing accuracy can be avoided, even when the above misalignment occurs.